The present invention relates to biofunctionalized quantum dots, which can be used, for example, in biological research, medical research, medical imaging, and medical therapy.
Quantum dots are small semiconductor particles that exhibit quantum confinement. See “Overview,” Quantum Dot Corp., (2003) http://www.qdots.com/new/technology/overview.html. A semiconductor has a characteristic band gap, which is the difference in energy between an electron in the valence band and an electron in the conduction band of the semiconductor material. When energy is applied to the material, for example in the form of a photon having a quantum of energy greater than or equal to the band gap, an electron can be stimulated to jump from the valence band to the conduction band. The missing electron in the valence band is referred to as a “hole”. See H. B. Gray, “Chemical Bonds,” (W. A. Benjamin, Inc., 1973), pp. 208-218. When an electron falls back into a “hole” in the valence band, a photon having a quantum of energy equal to the band gap, and thus a particular wavelength, can be emitted. Thus, materials in which high energy photons can cause electrons to jump into the conduction band, after which electrons can fall back into the valence band, emitting a photon, can exhibit the phenomenon of fluorescence. See A. E. Siegman, “Lasers,” University Science Books, 1986), pp. 6-15.
Quantum confinement refers to a phenomenon observed when the physical size of the semiconductor is smaller than the typical radius of the electron-hole pair (Bohr radius). In this case, the wavelength of light emitted through electron-hole recombination is shorter than the wavelength of light emitted by the semiconductor in bulk. The wavelength of light emitted by a semiconductor exhibiting quantum confinement can be termed the characteristic wavelength. Quantum dots can be made to fluoresce at their characteristic wavelength by exposing them to light having a wavelength shorter than the characteristic wavelength. The wavelength of light emitted is dependent on the size of the quantum dot: a smaller size results in a shorter wavelength. Therefore, the characteristic wavelength of a quantum dot can be “tuned” by adjusting the size of the quantum dot. Furthermore, techniques exist for producing quantum dots with narrow monodispersity in size, so that the light emitted from a number of quantum dots has a narrow bandwidth. See “Overview,” Quantum Dot Corp., (2003) http://www.qdots.com/new/technology/overview.html.
The essential part of a quantum dot is a nanocrystalline core, a semiconductor in a crystalline state which has a characteristic size of from about 1 to about 100 nm. Quantum dots used for their fluorescing properties can have a size range of from about 1 to about 10 nm. See “Anatomy”, Quantum Dot Corp., (2003) http://www.qdots.com/new/technology/dottech.html.
The quantum efficiency refers to the ratio of the number of photons emitted to the number of photons to which the quantum dot is exposed and which stimulate light emission.
To increase the quantum efficiency of a nanocrystalline core, and thereby enhance the intensity of fluorescence, the nanocrystalline core can be overcoated with a shell layer of a semiconductor material which has a band gap greater than the band gap of the nanocrystalline core. Bawendi et al, U.S. Pat. No. 6,306,610. A shell layer can also serve to protect the nanocrystalline core from the surrounding environment. If protection of the nanocrystalline core from the environment is important, but enhancement of quantum efficiency is not, a non-semiconductor material can be used for the shell layer. A quantum dot having both a nanocrystalline core and a shell layer can be referred to as a core/shell quantum dot.
Chemical groups, including chemical groups which have an effect on a biological system, can be bound to the surface of a quantum dot. This capacity to be functionalized, together with chemical stability and tunable fluorescing properties, makes quantum dots of great interest in the development of new materials and techniques for biological research and medical diagnosis. Furthermore, quantum dots are much less prone to photobleaching than many conventional dyes.
For most biological or medical applications, in order to be useful, a quantum dot must be rendered hydrophilic and have a biofunctional group attached to its surface. Chan and Nie linked mercaptoacetic acid to cadmium selenide core/zinc sulfide shell quantum dots. They bonded the protein transferrin to the linked mercaptoacetic acid groups by using ethyl-3-(dimethylaminopropyl) carbodiimide. Chan and Nie found that the transferin linked to the quantum dot was recognized by receptors on a cell surface. See Chan and Nie, “Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection”, Science, v. 281 (1998) p. 2016.
Akerman et al. used cadmium selenide core/zinc sulfide shell quantum dots coated with trioctylphosphine (TOPO), rendered them water soluble, and coated them with mercaptoacetic acid. Thiolated peptides were then linked to the surface of the quantum dots. Akerman et al. also made quantum dots in which thiolated polyethylene glycol and thiolated peptides were linked to mercaptoacetic acid coated quantum dots. They found that the peptide-functionalized quantum dots coupled with corresponding peptide receptors expressed by cells. See Akerman et al., “Nanocrystal targeting in vivo”, Proc. National Academy of Sciences, v. 99(2) (2002) p. 12617.
Larson et al. encapsulated a cadmium selenide core/zinc sulfide shell quantum dot within a amphiphilic polymer to render the quantum dot hydrophilic. They were able to image fluorescing quantum dots through the skin. Larson et al. suggested that the cadmium selenide core/zinc sulfide shell quantum dots leave the body before breakdown because there were no noticed toxic effects from the cadmium on mice into which they were injected. See Larson et al., “Water-Soluble Quantum Dots for Multiphoton Fluorescence Imaging in Vivo”, Science, v. 300 (2003) p. 1434.
Semiconductor nanocrystals can attach trioctylphosphine oxide (TOPO) as a ligand, rendering the semiconductor nanocrystals soluble in organic solvents such as chloroform and toluene, but not soluble in polar solvents such as water and ethanol. In an approach, a cadmium selenide core/zinc sulfide shell quantum dot was first coordinated with TOPO. Molecules in which mannose groups were covalently bonded to a phosphine oxide were then used to replace the TOPO groups on the cadmium selenide core/zinc sulfide shell, rendering the quantum dot hydrophilic. See Tamura et al., “Synthesis of Hydrophilic Ultrafine Nanoparticles Coordinated with Carbohydrate Cluster”, J. Carbohydrate Chemistry, v. 21(5) (2002) p. 445. However, it is doubtful whether the functionalized quantum dots produced were stable. In another approach, cadmium selenide core/zinc sulfide shell structures coordinated with TOPO were treated with a silathiane and mercaptosuccinic acid. The quantum dots were treated with a solutions of carboxymethyl dextran and of polylysine and treated with 1-ethyl-3-(3)-dimethylaminopropyl carbodiimide, which acts as a crosslinking agent. See Chen et al., “Synthesis of Glyconanospheres Containing Luminescent CdSe—ZnS Quantum Dots”, Nano Letters, v. 3(5) (2003) p581.
The applicants attempted to displace a TOPO layer on a cadmium selenide core/zinc sulfide shell quantum dot commercially available from Evident Technologies with a hydrophilic thiol compound using the modified phase-transfer procedure developed by Wang et al. See Wang et al., J. Am. Chem. Soc., v. 106 (2002) p. 2293. However, either the displacement was incomplete or the resultant functionalized quantum dots were fragile and did not survive mild ultrafiltration or dialysis and precipitated or flocculated shortly after the hydrophilic thiol compound was removed from the solution.
Bawendi et al. functionalized quantum dots with proteins and with oligonucleotides. The procedure used started with TOPO-capped cadmium selenide core/zinc sulfide shell quantum dots with which the proteins or oligonucleotides were linked. Bawendi et al., U.S. Pat. No. 6,306,610.
Gaponik et al. synthesized hydrophilic cadmium telluride core/cadmium sulfide shell quantum dots using an aqueous synthesis approach. In the approach, a cadmium salt and a mercapto-compound were mixed in an aqueous solution through which hydrogen telluride was bubbled. Cadmium telluride nanocrystals were formed which were capped at the surface with the mercapto compound. The mercapto-compound was linked to the cadmium telluride core through the sulfur atom. Thus, the cadmium telluride core was understood to be surrounded by a layer of sulfur atoms, which also were present deeper in the core, and which bonded to the cadmium atoms to form a cadmium sulfide shell layer. The hydrophilic cadmium telluride core/cadmium sulfide shell quantum dots exhibited good photostability; i.e., fluoresced over a long duration of illumination. Gaponik et al., “Thiol-Capping of CdTe Nanocrystals: An alternative to Organometallic Synthetic Routes”, J. Phys. Chem. B, v. 106 (2002) p. 7177.
For a preparation of quantum dots with biofunctional groups linked to their surfaces to be useful in biological research, medical diagnostic, and medical therapeutic applications, the quantum dots must fluoresce brightly, be hydrophilic, and be stable in water not containing excess biofunctional groups for prolonged periods of time.
Coupling of receptors to cell-surface saccharides mediates many relevant biological processes, including differentiation, motility, adhesion, tumor progression, and metastasis. Therefore, quantum dots functionalized with saccharides are of interest for biological research, medical diagnostic, and medical therapeutic applications. However, quantum dots suitable for such applications have up until now not been developed.
There thus remains a need for quantum dots which fluoresce brightly, have biofunctional groups linked to their surfaces, are hydrophilic, and are stable in aqueous solution. There is also a continuing need for quantum dots which have saccharides linked to their surfaces.